1. Field of the Invention
The present invention relates to a luminance and chrominance signal separation circuit for separating a composite color television signal into a luminance signal and a chrominance signal which may be used in a color television receiver.
2. Description of the Related Art
As is well known, in the NTSC color system, which is one of the existing television signal transmission systems, red, green, and blue primary-color signals are combined to produce a luminance signal representing the brightness of a scene and a chrominance signal conveying information representing the hues and saturation levels of the scene, and a composite color television signal is transmitted which includes the luminance signal and chrominance signal. The luminance signal is inherently transmitted for black-and-white television receivers and the chrominance signal is required to recover the primary-color signals and, when combined with the luminance signal, reproduces correct colors on color television receivers.
In the composite color television signal, the relationship between a color subcarrier for conveying the chrominance signal, and the luminance signal is set such that no signal interference occurs, as shown in FIG. 1. That is, the chrominance signal (C), with a subcarrier of 3.579545 MHz, is inserted in the upper portion of the frequency band of the luminance signal (Y) ranging from 0 to 4.2 MHz. To recover the primary-color signals from the composite color television signal in a color television receiver, therefore, it is necessary to separate the carrier chrominance signal from the luminance signal.
There are two methods for separating the chrominance signal from the luminance signal. In the first method, the luminance signal is derived by passing the composite color television signal through a 3.58 MHz trap by which the chrominance signal is rejected, while the chrominance signal is derived by passing the composite color television signal through a bandpass filter of 3.58.+-.0.5 MHz. The second method uses the so-called comb filter. This method utilizes the fact that in the NTSC color system the color subcarrier reverses in polarity between successive horizontal line periods (one horizontal line period is referred to hereinafter as 1H).
More specifically, the comb filter is comprised of a delay line for delaying the composite color television signal by 1H, an adder for adding the delayed composite signal and the non-delayed composite signal, a first attenuator for attenuating the output level of the adder by half, a subtracter for subtracting the delayed composite signal from the non-delayed composite signal, and a second attenuator for attenuating the output level of the subtracter by half.
Here it should be noted that the chrominance signal contained in the composite color signal during a certain horizontal line period is opposite in polarity to that during the subsequent horizontal line period. Hence the addition of the delayed composite signal and the non-delayed composite signal will cancel out the chrominance signals contained in the composite signals, but, since the luminance signal is not reversed in polarity in both the composite signals, it is not cancelled. As a result, only the luminance signal component is derived from the adder. In this case, however, the level of the luminance signal component is doubled due to addition by the adder, so the luminance signal component is halved by the first attenuator to obtain a correct luminance signal.
On the other hand, subtraction by the subtracter for the delayed and non-delayed composite signals will cancel out the luminance signal so that only the chrominance signal component passes through. In this case, subtraction by the subtracter will double the level of the chrominance signal component. The chrominance signal component is thus halved by the second attenuator to obtain a correct carrier chrominance signal.
With the conventional luminance and chrominance separation circuits using the trap and the comb filter, however, the following problems arise.
In the first method using the trap, a high-frequency components of the luminance signal as well as the chrominance signal will be eliminated. This will lead to the degradation of picture quality such as indistinctness of the details of a picture.
With the second method using the comb filter, as shown in FIG. 2, in a period of time (denoted by A) during which there is a correlation between the composite television signal (waveform a) and the 1H-delayed composite television signal (waveform b), the addition of the composite television signals will obtain the luminance signal (waveform c), from which the color sub-carrier signal is cleared, without rejecting high-frequency components of the luminance signal. However, where there is no correlation between the delayed and non-delayed composite television signals as in periods of time B and C, the chrominance signal component (designated by e) will remain in the luminance signal. As a result of this, dots will occur at the edge portions of a picture. Moreover, the luminance signal itself will be halved in amplitude level as shown at f in the periods B and C as compared with that in the period A. Hence a blunt picture will be produced which extends vertically.
In addition, the subtraction of the composite television signals in the period A during which there is a correlation between the composite television signal (waveform a) and the 1H-delayed composite television signal (waveform b) will produce the chrominance signal (waveform d) from which the luminance signal is cleared. Where there is no correlation between the composite television signals before and after 1H delay as in periods B and C, the luminance signal component (represented by g) will remain in the chrominance signal. As a result, cross color will be produced at the edge portions of a picture. Moreover, the chrominance signal itself will be reduced in amplitude level by half as compared with that in period A (refer to the portion designated by h). Hence a blunt image will be produced which extends vertically.
For the purpose of increasing the capability for separation of the luminance signal and the chrominance signal, another type of comb filter is known which performs a delay of 2H. Such a comb filter will make the drawbacks of the 1H-delay comb filter described above even more serious.
As described above, the conventional luminance and chrominance signal separation circuit which uses a trap has the problem of simultaneous elimination of high-frequency components of the luminance signal, in the attempt to eliminate the chrominance signal. Another conventional luminance and chrominance separation circuit which uses a comb filter has the problem of insufficient separation of the luminance signal and the chrominance signal in the case where composite television signals before and after 1H delay have no correlation.